I think they do it because they perceive that having a cabana has some sort of caché so to maximise their F experience they have to have one.

I first heard of the cabanas on here, where posters travelling F for the first time used to be told to make sure they booked a cabana as a matter of course. When I actually saw what they were I realised I had no need of one unless I wanted to sleep.

I agree that booking one and not using it is inconsiderate. I guess plenty of folk don't really worry about that any more.